The present invention relates to a method and apparatus for cleaning toner substance from the areas of a photoconductive drum outside the edges of an undersized copy sheet prior to removing the copy sheet from image transferring contact with the drum.
It is desirable that a dry process electrostatic copying machine be able to accommodate the largest commercial size of copy sheets. However, it is also desirable that the copying machine be able to operate using smaller sizes of copy sheets. Such a copying machine generally comprises a rotary photoconductive drum, a charger to apply an electrostatic charge to the drum and imaging means to radiate a light image of an original document onto the drum so that an electrostatic image of the original document is formed on the drum due to conduction in light areas of the image. A powdered toner substance is then applied to the drum to produce a toner image, and a copy sheet is pressed into contact with the drum to transfer the toner image to the copy sheet. An electric potential is preferably applied to the sheet to attract the toner particles from the drum. The copy sheet is then removed from the drum and the toner image is thermally fixed thereto.
If the original document and thereby a copy sheet utilized to produce a copy of the document is smaller than the maximum size, there will be areas outside the edges of the useful image area on the drum. A certain amount of toner substance will adhere to these areas during the developing step. If not removed, this toner substance will contaminate a transfer roller, a separating roller and the like and also the back surfaces of copy sheets of larger size used in a subsequent copying operation.
Various methods have been proposed in the prior art to overcome this problem, which include:
A. synchronizing the engagement of a separating roller with the copy sheet; PA1 B. changing the polarity of a bias voltage applied to a transfer roller; PA1 C. performing charging, developing and other operations only on the actual image area of the drum; and PA1 D. radiating light on the areas of the drum outside the image area to dissipate the electrostatic charge thereon.
These various methods all require intricate and expensive mechanisms, and are accurate in only one dimension. The difficulty of accurately synchronizing these prior art operations increases with the peripheral speed of the drum, and are extremely difficult to perform in high speed copying machines.
Another difficulty is encountered in cases in which the copy sheet is smaller than the original document, since the size of the original document is generally utilized for synchronization purposes by these prior systems.